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The Science of Keeping Cool

Learn how a cooling vest can help regulate body temperature in this video from NOVA: “Making Stuff Colder.” Host and technology columnist David Pogue explores what heat is and how it moves from areas of higher temperature to areas of lower temperature. Pogue first demonstrates how someone wearing military gear in hot conditions is susceptible to heat exhaustion, and then investigates the effectiveness of wearing a cooling vest as part of the gear. As Pogue exerts himself in a hot environment, his body temperature increases; however, when he puts on the cooling vest, the heat transfers to the chilled water in the vest, allowing him to stay cool.

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All matter is made of atoms and molecules that are constantly moving. A body's internal energy comprises two parts: the average kinetic energy of the atoms, which depends on the sum of their relative motions; and their mutual interaction energies. Temperature is a measure of the average kinetic energy of a substance's atoms and molecules—it does not depend on the number of particles. The transfer of thermal energy either causes a change in phase (for example, melting or boiling), or a change in temperature. When two bodies are placed in contact, heat will always flow from the body at higher temperature to the body at lower temperature.

The human body is constantly exchanging heat with the environment though the processes of conduction, convection, radiation, and evaporation: Conduction transfers heat through physical contact; convection transfers heat though the movement of air or water over the skin; radiation transfers heat as infrared light; and the body loses heat as the evaporation of moisture from the skin transfers heat from the body to the environment.

Normal human internal body temperature is about 98.6°F (37°C) at rest; however, temperatures within a few degrees (higher or lower) are considered normal. Humans have natural methods of thermoregulation—the ability to maintain core body temperature within normal limits and keep the body functioning properly. For example, sweating helps cool down the body through the process of evaporation, while shivering increases heat production in muscles to help the body warm up when it is cold. However, if the core temperature reaches a mark that is significantly above or below normal, serious injury or death could result.

When the body produces or absorbs more heat than it releases, above-normal body temperature occurs. People who work in hot and humid conditions are especially at risk for heat-related illnesses (such as heat cramps, heat exhaustion, and heat stroke) because the body is not able to lose heat effectively. Health effects from heat stress include headache, dizziness, fatigue, nausea, loss of coordination, difficulty breathing, and rapid pulse. People who work in hot environments or wear heavy protective clothing (such as military personnel or firefighters) could benefit from technology that helps the body get rid of excess heat. For example, some cooling vests work by circulating chilled water near the skin to remove heat from the body, which helps keep the core temperature down. Personal cooling vests can also be beneficial for athletes or for people who are heat sensitive (such as multiple sclerosis patients).